Isotopically altered optical fiber

ABSTRACT

An isotopically-altered, silica based optical fiber is provided having lower losses, broader bandwidth, and broader Raman gain spectrum characteristics than conventional silica-based fiber. A heavier, less naturally abundant isotope of silicon or oxygen is substituted for a lighter, more naturally abundant isotope to shift the infrared absorption to a slightly longer wavelength. In one embodiment, oxygen-18 is substituted for the much more naturally abundant oxygen-16 at least in the core region of the fiber. The resulting isotopically-altered fiber has a minimum loss of 0.044 dB/km less than conventional fiber, and a bandwidth that is 17 percent broader for a loss range between 0.044-0.034 dB/km. The fiber may be easily manufactured with conventional fiber manufacturing equipment by way of a plasma chemical vapor deposition technique. When a 50 percent substitution of oxygen -18 for oxygen-16 is made in the core region of the fiber, the Raman gain spectrum is substantially broadened.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. Application Ser. No.10/322,148, filed Dec. 18, 2002, now U.S. Pat. No. 6,810,197 whichclaims the benefit of priority under 35 U.S.C. § 119(e) of U.S.Provisional Application Ser. No. 60/341,256 filed on Dec. 20, 2001.

FIELD OF THE INVENTION

This invention generally relates to an isotopically altered opticalfiber, and is specifically concerned with a silica fiber enriched withthe heavier isotopes of oxygen and/or silicon in order to reducetransmission losses and to increase bandwidth.

BACKGROUND OF THE INVENTION

Optical loss is a limiting factor in the design and construction ofoptical networks and links, which typically include hundreds ofkilometers of silica-based optical fiber. Consequently, a reduction inthe loss of the fiber by even hundredths of decibels per kilometer wouldhave a significant impact on the performance of such networks. Opticallosses in silica fibers are predominantly caused by two factors,including (1) Rayleigh scattering, which falls off as a function of 1/λ⁴and which dominates for shorter wavelengths, and (2) infrared absorptionby the silica, which dominates for longer wavelengths. As is well knownin the art, the product of these two forms of optical loss falls to aminimum at a wavelength of approximately 1560 nm. Consequently, mostoptical signals are transmitted at a bandwidth centered around 1560 nmin order to minimize transmission losses. At a bandwidth between1510-1610 nm losses typically vary between a minimum of 0.189 dB/km anda maximum of 0.200 dB/km. As small as this may seem, such a loss ratestill translates into a 50 percent loss of signal over a distance of 15km, which is quite significant when one considers that some networks(such as the one traversing the Atlantic Ocean) are over 6,500 km long.

While there have been various attempts in the past to develop an opticalfiber with lower transmissivity losses, the high costs associated withthe manufacture of such fibers has prevented them from enjoyingwidespread use. So called silica core fibers, e.g., those having anundoped silica core and a fluorine doped cladding, are known which havereduced losses on the order of 0.151 db/km at 1550 nm are known.However, it is well known that silica core fibers are much moredifficult to manufacture than fibers having cores which include index ofrefraction-altering dopants. Other types of fibers with potential forlow loss are known which employ a non-silica chemistry. However, thematerials used in such fibers are far more expensive than silica, andcannot be drawn and worked on a commercial scale without the developmentof completely different kinds of manufacturing equipment than ispresently in use.

SUMMARY OF THE INVENTION

The invention relates to an optical waveguide comprising multipleisotopes of a same chemical element in relative proportions sufficientlychanged from a naturally occurring proportion of the isotopes such thatthe optical losses are reduced in the vicinity of the standard 1510-1610nm bandwidth, the bandwidth is substantially increased, and the Ramanspectrum is broadened. The waveguide preferably is an optical fiber.

In one embodiment, the waveguide includes a light conducting core regioncomprised of silica glass, and at least a portion of the oxygen in thesilica is comprised of multiple isotopes of oxygen in relativeproportions which are changed from a naturally occurring proportion ofoxygen. In particular, oxygen-18 makes up greater than 20 mole percent,more preferably greater than 50 mole percent, even more preferablygreater than 70 mole percent, and most preferably greater than 80 molepercent of the total amount of oxygen in the core region. At the sametime, in any of the embodiments disclosed herein wherein the amount ofoxygen-18 is increased in the waveguide over that which is naturallyoccurring, the amount of oxygen-17 is likewise increased, preferably atleast 5 percent, more preferably at least 10 percent, and mostpreferably at least 15 percent of the amount of oxygen-18 employed inthe fiber. Consequently, for example, when the oxygen-18 is greater than50 mole percent of the total oxygen present, it is preferable that theoxygen-17 be greater than about 2.5 mole percent, more preferably atleast 5 mole percent, and more preferably greater than about 7.5 molepercent, of the total molar oxygen content.

As the losses due to Rayleigh scattering remain about the same forwavelengths in the range of about 1500 to 1800, nm while the losses dueto infrared absorption in this range are reduced, the net effect is thata new minimum transmission loss of approximately 0.145 dB/km occurs nearwavelengths of about 1670 nm. Because the loss curve with respect towavelength is flatter in the vicinity of the new minimum, bandwidth overa variation of 0.010 dB/km (i.e., in a loss range of between 0.145 and0.155 dB/km) is increased 17 percent over the 100 nm bandwidth ofconventional germanium-doped fiber. If a maximum loss rate of about 0.19dB/km can be tolerated (which is the minimum loss of the lowest lossgermanium-doped fiber presently commercially available), bandwidth canbe increased 100 percent. Finally, if a maximum loss rate of 0.200 dB/kmcan be tolerated (which is the loss rate of conventional fibers)bandwidth can be increased 275 percent.

While the resulting reduction of transmission losses between 0.034 and0.044 dB/km would not appear to be large, such a reduction wouldtranslate into substantial savings, particularly in long distancetransmission networks. For example, when conventional fiber is used toform a network traversing the Atlantic Ocean, the optical losses whichoccur necessitate signal regeneration stations for every 125 km offiber. With the reduction in transmission losses of only between 0.034and 0.044 dB/km, such regeneration stations are required only every 156km. This would result in a net reduction of 11 regeneration stations. Assuch ocean based stations cost approximately one million dollars apiece, the net savings in a transatlantic transmission line amounts toover $11 million.

Another embodiment of the invention relates to an improved Raman gainfiber, the full width half maximum of the Raman gain spectrum in suchfibers may be increased at least 5 percent over fibers made usingnaturally occurring oxygen. In this particular embodiment of theinvention, the enrichment of either or both of the oxygen or silicon inthe core region of the fiber with one of the heavier isotopes of eitheroxygen or silicon does not have to amount to a complete substitution, asit may be in other embodiments. Rather, if oxygen-18 is used as theenriching isotope, only about half of the oxygen in the core region needbe oxygen-18, the balance being oxygen-16. The reason for such anincomplete substitution is that a complete substitution results in acomplete shifting of the Raman gain spectrum, whereas a roughly 50percent substitution of oxygen-18 for the oxygen-16 has the moreadvantageous effect of broadening the Raman gain spectrum by thesuperposition of an ordinary Raman gain spectrum with the shifted Ramangain spectrum which would be obtained if only oxygen-18 were used in thesilica forming the core region. For example, in one embodiment, the coreof the Raman gain fiber is comprised of layers of glass which alternatebetween layers which are enriched with oxygen-18, as described above,and layers of glass which are not enriched beyond that which is presentin naturally occurring oxygen. Consequently, for embodiments involvingRaman gain fiber, the level of substitution of oxygen-18 for oxygen-16is preferably between about 30-70 mole percent, more preferably betweenabout 40-60 mole percent, and most preferably between about 45-55 molepercent

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a silica-based optical fiberillustrating the relative size of the core region CR and the cladding CLand the various regions of the fiber surrounding the core region CRwhere isotopic substitution of oxygen-18 and/or silicon 30 is preferred;

FIG. 2 is a graph comparing attenuation versus wavelength for germanium(Ge) doped silica fibers containing oxygen-16 and oxygen-18;

FIGS. 3A-3D illustrate exemplary refraction index profiles of a standardsingle-mode fiber, a large effective area fiber, a specializedsingle-mode fiber, and a dispersion-compensating fiber, and whereoxygen-18 enrichment preferably occurs within these profiles;

FIG. 4 compares the attenuation between a conventional optical fiber(having predominantly oxygen-16) and a fiber of the invention enrichedwith oxygen-18 on a logarithmic scale;

FIG. 5 compares the fibers referred to in FIG. 4 on a linear scale;

FIG. 6 also compares conventional and oxygen-18 enriched optical fiberson a linear scale, and further illustrates the broadening oftransmission bandwidth associated with oxygen-18 enriched fibers;

FIG. 7 illustrates how the index of refraction varies with differentoptical wavelengths for a conventional fiber, a fiber having a coreisotopically enriched with oxygen-18 in a cladding without isotopicenrichment, and a non-isotopically enriched core with a claddingenriched with oxygen 18;

FIG. 8 is a comparison of the group index of refraction for the sameoptical fibers as described with respect to FIG. 7;

FIG. 9 compares material dispersion between conventional single modeoptical fibers and the same optical fibers having either or both thecore and the cladding enriched with oxygen-18;

FIG. 10 compares with waveguide dispersion vs. wavelength of the sametype of fibers compared in FIG. 9 with respect to waveguide dispersion;

FIG. 11 compares the profile dispersion vs. wavelength of the same typeof fibers compared in FIG. 9;

FIG. 12 compares the total chromatic dispersion vs. wavelength of thesame type of fibers compared in FIGS. 9-11;

FIG. 13 illustrates minimum loss as a function of the percentage ofoxygen-18 in the core region of the fiber;

FIG. 14 is a graph of the wavelength corresponding to the minimum lossas a fraction of the amount of oxygen-18 in the core region of thefiber,

FIG. 15 is a graph of the increase in bandwidth as a fraction of theamount of oxygen-18 in the core region of the fiber;

FIG. 16 is graph of the attenuation of optical fiber as a function ofpercentage of oxygen-18 in the core region of the fiber,

FIG. 17 compares the attenuation of a wavelength of conventional fiberwith fibers of the invention, and

FIGS. 18A and 18B are complementary graphs showing relative amounts ofoxygen-16 and oxygen-18 with respect to a radial position from the coreregion of a cane from which the fibers of the invention are drawn.

FIG. 19 compares measurements of the Raman cross-section of experimentalsingle-mode optical fibers containing either essentially 100 percentoxygen-16 or essentially 100 percent oxygen-18.

FIG. 20 compares the shifting and broadening of the theoretical Ramancross section for optical fibers containing various mixtures ofoxygen-16 and oxygen-18.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates one embodiment of the invention comprising a silicaoptical fiber having core region CR surrounded by a cladding CL, whereinthe core region has been enriched with one of the heavier isotopes ofeither oxygen or silicon. The stable isotopes of silicon have atomicweights of 28, 29, and 30 with natural abundances of 92.23 percent, 4.67percent, and 3.10 percent, respectively. In the case of oxygen, thestable isotopes have atomic weights 16, 17, and 18, with abundances of99.762 percent, 0.038 percent, and 0.2 percent, respectively. In bothcases, the most abundant isotope constituting silica (SiO₂) are thelightest stable isotopes. Stable isotopes are not radioactive and arestable over time and temperatures. The chemical behavior of theaforementioned isotopes are identical to their more abundant, lightercounterparts. Hence, as described in detail hereinafter, the chemicalsubstitution of heavier isotopes such as silicon-30 or oxygen-18 fortheir lighter counterparts is easily implemented.

In the preferred embodiment of this invention, a substantial percentageof the oxygen-16 in the core region CR of the fiber is replaced withoxygen-18, or in other words, the amount of oxygen-18 in the fiber isgreater than that which occurs naturally. There are two reasons forthis. First, oxygen-18 may easily be integrated into the core region ofa fiber via the same type of plasma vapor deposition techniquespresently known in the art and disclosed in such references as thePartus et al., U.S. Pat. No. 5,692,087. Secondly, oxygen-18 iscommercially available as it is presently being used for certain medicalapplications. However, it is presently expensive, costing about $550/gm.

Fortunately, as the vast majority of the light conducted by opticalfibers is conducted through the core region, it is not necessary toreplace the oxygen-16 in the cladding of the fiber in order to obtainenhanced optical performance. Specifically, it is estimated thatapproximately 80 percent of the modal volume of light is conductedthrough the core region illustrated in FIG. 1 that circumscribes acentral axis C. As the core region in a standard, single mode fiber hasa diameter of only about 9 microns, and as the overall diameter D of thefiber is approximately 125 microns, it follows that all of the benefitsof the invention may be realized for 78 percent of the transmitted lightif only 0.51 volume percent of the silicon dioxide of the fiber isenriched with oxygen-18. Consequently, in a preferred embodiment of theinvention, the volume percent of the oxygen and the silicon dioxide ofthe fiber which is enriched with oxygen-18 is less than 4 percent, morepreferably less than 3 percent, and most preferably less than 2 volumepercent. Of course, greater benefits may be realized if oxygen-18 issubstituted for oxygen-16 in the cladding immediately adjacent to thecore region CR of the fiber. For example, if a circular area 1.5 timesthe area of the core region CR is so enriched, 87 percent of the modalvolume of the light will “see” oxygen-18 enriched silica if an area 2.25times the area of the core region CR is enriched, at least about 95percent of the light will “see” oxygen-18. Finally, if an area fourtimes the area of the core region CR is enriched, virtually all of thelight (99 percent) will traverse oxygen-18 enriched silica. Of course,such expansions of 1.5, 2.25, and 4.0 times the area of the core regionCR will require the percentage of enriched silica to increase to 0.77percent, 1.15 percent, and 2.04 percent, of the total silicarespectively. In view of the high cost of oxygen-18 (presently about$500.00 per liter), the inventors contemplate a preferred embodimentwherein a central, circular area having a diameter of about 1.25 timesthe diameter of the core region CR is isotopically enriched withoxygen-18. Such proportioning would insure the capture of at least 90percent of the light transmitting region of the fiber, while requiringthat only about 0.8 percent by volume of the silica of the fiber beisotopically enriched with oxygen-18.

FIG. 2 summarizes the lowered optical attenuation associated with theaforementioned substitution of oxygen-18 for oxygen-16 at least in thecore region CR. As has been previously indicated, optical losses atshorter wavelengths are due predominantly to the scattering of lightcaused by fluctuations in the density of the glass. This phenomenon iscalled Rayleigh scattering. At longer wavelengths on or around 1550 nm,Rayleigh scattering losses taper off and losses are dominated byinfrared absorption of the silica molecules. In conventional silicafibers, the minimum loss of 0.189 dB/km occurs at approximately 1550 nm.By contrast, as a result of the heavier oxygen-18 atoms lowering thefrequency of the infrared absorption of the silica molecules, theminimum loss falls to 0.145 dB/km at a wavelength of about 1670 nm. Aswill be discussed in more detail with respect to the graph of FIG. 6,this lowered loss is accompanied by an increase in the usable bandwidthof the fiber due to the flatter nature of the attenuation curve (shownas a solid line) of the inventive fiber. Finally, as will be discussedwith respect to FIGS. 19 and 20, the partial substitution of heavierisotopes of either the oxygen or the silicon in the core region of thefiber advantageously broadens the Raman gain spectrum of the fiber thusfacilitating Raman amplification.

FIGS. 3A-3D illustrate the extent to which the core region CR of anoptical fiber might be enriched with oxygen-18 relative to its indexprofile in order to obtain the advantages of the invention. FIG. 3Aillustrates the index profile of a standard, single mode fiber. Here,the core region containing the germanium dopant is enriched withoxygen-18. For standard step index single mode fibers such as Corning'sSMF-28, this amounts to an enrichment of the fiber of at leastapproximately 4 microns on either side of central axis C to capturesubstantially all of the flattened bell curve of germaniumconcentration. Preferably, the SiO₂ is enriched a distance into thecladding which is adjacent to the core region, for example, a distancegreat enough to reach the SiO₂ clad region. Such an enrichment patternshould result in at least 80 percent of the transmitted light “seeing”the oxygen-18 enriched silica FIGS. 3B-3D illustrate various segmentedcore refractive index patterns. FIG. 3B is the index profile of a largeeffective area fiber. Here, oxygen-18 enrichment should extend out tojust beyond the two side peaks of germanium dopant concentration. Again,this would typically amount to providing an oxygen-18 enrichment of thesilica between about 4 and 5 microns from the central axis C of thefiber. FIG. 3C illustrates a specialized single mode fiber, or avariation of the fiber index illustrated in FIG. 3B. Here, the sameprincipal applies and that part of the core region CR encompassing bothside peaks of germanium dopant concentration should likewise be enrichedwith oxygen-18. Again, this amounts to an enrichment of the core regionCR a radial distance of between 4 and 5 microns from the central axis C.FIG. 3D illustrates the index profile of a dispersion compensatingfiber. Here, only that portion of the core region CR containing thecentral peak might be enriched with oxygen-18 since enrichment of theareas containing the side peaks would increase the deleterious effectsof higher order mode transmission. For certain dispersion compensatingfibers, such as the one illustrated in FIG. 3D; it may be desirable tokeep the attenuation higher in the regions of the outer ring, because bydoing so, these regions will attenuate more than the inner regions ofthe core, helping to decrease the higher order modes which travel inthis region, which in turn will help decrease signal noise generated bymultipath interference. While the transitions between oxygen-18 andoxygen-16 have been schematically illustrated as being sharptransitions, the invention encompasses designs wherein smoothertransmissions between the enriched and unenriched silica aredeliberately manufactured in order to create an 0-18/0-16 gradient of amicron or two. Such a gradual transition may be used to advantageouslycontrol the diffusion which results from the difference in the index ofrefraction between ordinary silica and isotopically enriched silica

FIGS. 4, 5, and 6 illustrate, in more detail, the advantageous losscharacteristics of the isotopically altered fibers of the invention.FIG. 4 parallels the graph of FIG. 2 with the exception that ultravioletlosses are included. FIG. 5 illustrates via a linear scale the relativeattenuation between the fibers formed from ordinary silica havingpredominantly oxygen-16, and fibers of the invention having a coreregion CR enriched with oxygen-18. FIG. 6 is similar to FIG. 5, with theexception that the bandwidth expansion advantages of the invention areclearly illustrated. In particular, for a decibel loss differential of0.02 dB/km (i.e., the same differential tolerated in standard silicafiber), the bandwidth of the fiber of the invention is 17 percent longerthan the approximately 100 nm bandwidth associated with conventionalfiber. If a maximum loss of 0.175 dB/km can be tolerated (which is thesame loss associated with the lowest-loss silica fiber presentlycommercially available) then the bandwidth can be extended approximately100 percent as illustrated by the dotted line. Finally, if the sameamount of loss that can be tolerated is the same as the maximum lossassociated with conventional fiber (i.e., 0.200 dB/km) then thebandwidth expands approximately 270 percent. Although current opticalnetworks do not use such a broad bandwidth, future networks may requirethe additional bandwidth to add more wavelength-division multiplexedchannels to handle the growing global bandwidth consumption. Such broadbandwidth may also be attractive in networks that use coarsewavelength-division multiplexed in which few channels are broadly spacedin wavelength. Coarse wavelength division multiplexing is used inshorter-haul metro or access networks.

FIG. 5 illustrates the relative effect that can be achieved by replacinga portion of the oxygen-16 in the silica based fiber with oxygen-18. Inthe embodiment illustrated, 100 percent of the oxygen in the silicalattice has been replaced by oxygen-18. Clearly, the attenuation of theoxygen-18 fiber is much lower overall than that of the oxygen-16 fiber.For example, incorporation of oxygen-18 into the silica based fiberwould result in an unprecedented increase in bandwidth, greater than150, more preferably greater than 200, and most preferably greater than250 nm of bandwidth having an attenuation of less than 0.2 dB/km.Alternatively, an even lower attenuation bandwidth could be utilized,i.e., greater than 100 nm, more preferably greater than about 200 nmbandwidth having an attenuation less than 0.18 dB/kn. FIG. 5 illustratesthe comparison between a conventional step index germania doped silicabased fiber (similar to Corning's SMF-28), and an oxygen-18 dopedversion of the same fiber. As can be seen in FIG. 5, doped withoxygen-18 enables a step index germania doped single mode fiber havingan attenuation less than 0.18, more preferably less than 0.17, and mostpreferably less than 0.16 dB/km, for example at about 1670 nm.

FIGS. 7-12 are various graphs illustrating differences in the index ofrefraction and dispersion between conventional silica, germania doped,step-index single mode fibers (e.g. such as Corning's SMF-28™ and thesame fiber which has been isotopically enriched in accordance with theinvention. FIG. 7 compares how the index of refraction varies overwavelength for a conventional single mode fiber, and fibers of theinvention having either an isotopically enriched core and conventionalcladding or conventional core region and an isotopically enrichedcladding. FIG. 8 graphically compares the same quantities for the samefibers in terms of group index (i.e., index of refraction with respectto pulse signals as opposed to wave fronts). Both of these graphsillustrate the fact that silica enriched with oxygen-18 has a higherindex of refraction, and that the more such enriched silica is presentin the fiber, the more effect the higher index of refraction forenriched silica has on the fiber as a whole. There are two majorconsequences of these differences in the index of refraction. First, thesignificantly higher index of refraction of isotopically enriched silicamay advantageously be exploited to lowers costs by reducing the amountof germanium or other dopant used to create the index difference betweenthe core region and the cladding necessary to effect total internalreflection. Such a lowering in the concentration of dopant will have theadvantageous effect of reducing the amount of Rayleigh scattering, hencefurther lowering the losses created with the fiber of the invention.Additionally, the lower dopant concentration will provide a lowerviscosity mismatch between the core and cladding leading to lowerstress-induced losses. Secondly, when only the core region or thecladding of the fiber is isotopically enriched, the resulting differencein index of refraction between the core and cladding offers the fiberdesigner a new parameter by which to manipulate or correct the resultingdispersion characteristics of the fiber. Such differences in dispersionare most evident in graphs such as FIG. 11 and FIG. 12. FIGS. 13, 14,and 15 illustrate the minimum loss, the wavelength of minimum loss, andthe amount of bandwidth associated with attenuation losses of less than0.20 dB/km as a function of the percentage amount of oxygen-18 presentin the core region. The curves in these graphs illustrate the desire forreplacing substantially all of a naturally occurring isotope of silicaor oxygen with a heavier isotope such as oxygen-18 if the benefits ofthe invention are to be maximized, as each of these curves is nonlinear.In particular, FIG. 13 illustrates that if only 50 percent of the oxygenin the core region is replaced with oxygen-18, then the attenuation losswill fall to only about 0.174 dB/km. Stated differently, a 50 percentreplacement of oxygen-16 with oxygen-18 results in only 30 percent ofthe maximum reduction of attenuation that can be achieved with 100percent substitution. These same proportions apply with respect toadvantages in increased bandwidth, as borne out by FIG. 15.Specifically, if only 50 percent of the oxygen-18 in the core regionsilica is replaced with oxygen-18, bandwidth expansion is only 30percent of the maximum expansion available when a 100 percentsubstitution is implemented. In view of these nonlinear relationships ina preferred embodiment a manufacturing process is chosen which iseffective in replacing substantially all of the oxygen-16 in the coreregion with oxygen-18. However, due to the practical difficulty involvedwith some manufacturing processes, it may be difficult to achievesubstitution of substantially all of the oxygen-16. Consequently, theoxygen-18 substitution should be greater than 20 mole percent, morepreferably greater than 50 mole percent, even more preferably greaterthan 70 mole percent, and most preferably greater than 80 mole percentof the total amount of oxygen-16 in the core region.

At the same time, in the preferred embodiments of the invention, theamount of oxygen-17 is at least 5 percent, more preferably at least 10percent, and most preferably at least 15 percent of the amount ofoxygen-18 employed in the fiber. Consequently, for example, when theoxygen-18 is greater than 50 mole percent of the total oxygen present,it is preferable that the oxygen-17 be greater than about 2.5 molepercent, more preferably at least 5 mole percent, and more preferablygreater than about 7.5 mole percent, of the total molar oxygen content.

FIG. 16 illustrates that, for a conventional germania doped step indexfiber, the amount of attenuation that occurs when a 100 percentoxygen-18 substitution is achieved is lowest at a wavelength of 1670 nm.As this wavelength of minimum attenuation is somewhat higher than theminimum attenuation wavelength associated with conventional fibers(which is approximately 1560 nm), this graph illustrates thedesirability of utilizing the fiber of the invention with opticalcomponents designed to operate near the “red” end of the opticalcommunication spectrum. For example, in one embodiment, the fiber isemployed in a telecommunications system which comprises at least atransmitter and a receiver, and the transmitter and receiver areconfigured to operate at a wavelength greater than about 1625, morepreferably greater than about 1635.

FIG. 17 is a graph of attenuation as a function of wavelength for a stepindex, germania doped single mode fiber of the invention produced by wayof a plasma vapor deposition process. As is evident from this graph, themeasured difference in loss between a conventional fiber (upper line)and the fiber of the invention (bottom dashed line) was 0.044 dB/km inaccordance with the result predicted by the previous graphs.

FIGS. 18A and 18B graphically illustrate radial profiles, in molepercent vs. radial position, of the oxygen isotopes present in the coreregion of an optical fiber core cane comprised of silicon dioxideproduced from the aforementioned PCVD process. In these graphs, theoverall core cane diameter was 16 mm in diameter, while the core regionwas 2 mm in diameter. As is evident from both of these graphs, a nearly100 percent substitution of oxygen-18 for oxygen-16 was accomplished fora radius of approximately 1000 microns. This 1000 microns willcorrespond to a radius of approximately 4 to 5 microns in the drawnfiber. Hence, the PCVD process may be used to produce fibers inaccordance with the invention having core regions wherein the oxygen-16is greater than 50 percent, preferably greater than 60 percent, morepreferably than 75 percent, and most preferably greater than 93 percentreplaced with oxygen-18, thereby realizing substantially all of thepotential benefits of the invention illustrated in FIGS. 6, and 13-15.It is believed that these same amounts of oxygen-18 substitution can beachieved using MCVD techniques. As is the case with all embodimentsdisclosed herein, preferably, the oxygen-18 containing layers of glassalso contain oxygen-17, in the relative concentrations described above(at least 5 percent, more preferably at least 10 percent, and mostpreferably at least 15 percent of the amount of oxygen-18 employed inthe fiber).

In order to achieve the previously referred-to core cane of isotopicallyenriched silica, the following PCVD process is used. First, a flow ofgas including at least one silica glass-forming precursor (SiCl₄) and agermania dopant (GeCl₄), and an oxidizing agent containing substantially100 percent oxygen-16 is directed, under low pressure, through aheat-resistant silica glass tube (outer diameter=25 mm) which will formthe cladding of the optical fiber core cane. Simultaneously, the tube isheated with a plasma heat source, which may be an oxygen/argon plasmaheat source. The plasma zone is traversed along and around the tube,causing SiO and GeO to react and deposit on the innermost walls of thetube as fully consolidated glass. The traversing of the plasma zone iscontinued and the levels of different components in the gas stream arevaried so as to effect changes in the compositions of the outermostglass layers, which will eventually become part of the cladding of thefiber. Several hundreds of layers were deposited. As the layers of glassthat are to become the core region of the core cane are approached, thegas stream composition flowing through the tube is adjusted such thatsubstantially 100 percent of the oxygen is oxygen-18. To accomplishthis, 99 percent isotopically pure oxygen-18 was flowed into the tube ata point to coincide with the core region and to extend 100 μm radiallybeyond that region. As can be seen in FIGS. 18A and 18B, the oxygen-18distributed exactly as intended. At the same time, the amounts ofnecessary dopant (in this case germanium) are adjusted. After all of thedesired layers of glass have been deposited, the resulting tube of glassis collapsed to form the optical fiber core cane, which consisted ofboth the doped core region and a small portion of the cladding.Additional undoped SiO₂ soot was then added to the outside of the corecane to produce a fiber preform. Then the preform was consolidated anddrawn into optical fiber. With such processing, the core-cane materialbecomes center 40 micron diameter region of the optical fiber which, ofcourse, contains the smaller 10 micron diameter core region. Oneadvantage to using the PCVD process is the excellent utilization of theisotope dopant, to amounts as high as 90-100 percent utilization of theoxygen-18 dopant. In this demonstration we achieved 50 percent oxygenutilization.

In one preferred embodiment of the invention, a quantity of silica basedsoot is deposited using oxygen-16, and then a portion of the oxygen-16in the fiber soot blank is converted to oxygen-18 (and preferably oxygen17 as well) by doping of the fiber soot blank prior to consolidation ofthe soot into consolidated glass. For example, in the conventionaloutside vapor deposition process, soot is deposited onto a bait rod. Inthe preferred embodiment of the present invention, the core region isfirst deposited onto the bait rod. For example, in the case of a stepindex single mode fiber, a germania doped portion of silica soot can bedeposited onto bait rod to form a porous soot blank. The bait rod isthen removed from the soot blank to leave a hole down the center of theporous soot blank. The soot blank is then suspended in a consolidationfurnace, and appropriate gases are flowed through and around the blankto remove hydroxyl groups (e.g. chlorine is typically used to dry thesoot). Either prior to or subsequent to the drying step, the soot blankis introduced to oxygen that is enriched with oxygen-18 (and preferablyoxygen 17 as well) isotope. As the oxygen diffuses through the porousbody, an oxygen atom incorporated in the silica matrix may undergo asurface-mediated exchange with the gaseous molecular oxygen. On amicroscopic scale such exchanges will occur indefinitely butmacroscopically the process will reach a thermodynamic equilibrium atwhich the ratio of isotopes in the gas phase will be equal to the ratioof isotopes in the solid phase. This process is done at temperatures andpressures which allow for efficient exchange of oxygen-18 for oxygen-16within the soot particles that make up the blank. Preferred temperaturesfor such processes are greater than 900, more preferably greater than1000, and most preferably greater than 1100 C. For a temperature of 1162degrees Celsius and pressure of 1.0 atmosphere this equilibrium wasdemonstrated to be reach in approximately 110 minutes. The flow ofoxygen-18 is stopped when sufficient oxygen-18 has been delivered todisplace the oxygen-16 within the core region inside the blank.

After exchange of the oxygen-18 for the oxygen-16 in the soot particles,the blank is heated and consolidated into a clear glass preform. Insubsequent processing steps, the centerline hole is closed to form acore cane, as is known in the art. This consolidated glass core region,which consists of the germania doped portion and preferably a smallportion of the cladding would consist largely of oxygen-18 doped SiO₂.This core cane could then have additional cladding soot deposited ontothe outer periphery, after which this cladding soot could be dried andconsolidated to form a optical fiber preform which can be drawn into anoptical fiber. If desired, this additional soot could likewise beexchanged with oxygen-18. However, in the preferred embodiment only thecore portion and a optional small adjacent region of the cladding isconverted to oxygen-18.

Various gases could be employed to dope the soot preform of oxygen-18.For example, the mixture of oxygen-18, oxygen-17 and oxygen-16,oxygen-18 enriched air, pure or substantially pure oxygen-18 gas, D₂¹⁸O, H₂ ¹⁸O, and mixtures thereof could be employed. In one preferredembodiment, D₂ ¹⁸O and/or H₂ ¹⁸O is employed as the doping or exchangegas. For example, we have found that the exchange of H₂ ¹⁸O with silicasoot reaches equilibrium in less than about 7 minutes at 1162 Celsius.The soot is preferably exposed to greater than 50 atom percentoxygen-18, more preferably greater than 95 atom percent oxygen-18,during the oxygen-18 conversion step. Preferably, the oxygen-18containing gases also contain oxygen-17, in the relative concentrationsdescribed above (at least 5 percent, more preferably at least 10percent, and most preferably at least 15 percent of the amount ofoxygen-18 employed in the fiber). The time required for isotopicexchange with the soot is preferably minimized by using hightemperature, but preferably the temperature is not so hot that sinteringof the soot occurs since the resulting reduction in porosity would limitisotopic exchange.

Preferably, in any manufacturing operation which employs use ofoxygen-18 in a fashion in which less than 100 percent of the oxygen-18is used up during the process, the process employs a means for recyclingthe unused oxygen-18 containing dopant or exchange gas back into themanufacturing process. For example, in the above process described todope oxygen-18 during the consolidation of the soot, the waste oxygenmay be recovered from the consolidation furnace and fed back into theconsolidation furnace at a later time or continuously throughout thedoping process. Preferably the waste oxygen is cycled to an oxygenpurifier to produce a nominally pure oxygen stream. If necessary thepurified oxygen may be fed to an isotopic separation unit which furtherenhances the percentage of oxygen-18 in the oxygen stream, therebyproducing a nominally pure oxygen-18 stream to be fed back into theconsolidation furnace. The recycled oxygen-18 stream may be fed to aholding tank, back into the manufacturing process, or vented if it doesnot meet a required purity level. In the preferred embodiment, which isemployed to dope oxygen-18 into the OVD formed soot core cane prior toconsolidation, waste oxygen is separated from the exhaust gas, testedfor purity (with a residual gas analyzer, for example) and thenrecirculated back to the input gas stream where it is combined with thefeed stream of oxygen. In an MCVD or PCVD process, the oxygen in theexhaust gas should contain the same isotopic ratio as that of the inputstream. Thus, if 100 percent oxygen is used in the input stream there islikely no need to do an isotopic separation of the exhaust oxygen.However in an OVD or VAD process, in which soot is to be doped usingoxygen-18, there will likely be a need for isotopic separation becauseof the dilution of oxygen-18 from the oxygen-16 present in the ambientair, the burner oxygen, and any oxygen-16 contained in the burner fuel(CO₂, for example). Removal of the fuel oxygen may be simplified becauseit will likely be in the form of water. The recirculation is notnecessarily continuous and may involve a temporary holding tank in whichenriched oxygen is stored. The non-isotopic oxygen separation mayinclude solid and fluid separation, cryogenic separation, chemicalabsorption, catalytic reaction, absorption systems, membrane separation,and pressure or thermal swing absorption systems, and combinationsthereof. For example, the oxygen separation might consist of aparticular filter for separating the soot from the exhaust gases; atreatment of the soot with carbon to generate C—¹⁸O₂ from which theoxygen-18 will be subsequently released (by solid oxide electrolysis,for example); and a cryogenic separation of oxygen gas from exhaustgases. The oxygen-18 containing gas which is recycled preferably alsocontains oxygen-17 when it is returned to the optical fibermanufacturing process, in the relative concentrations described above(at least 5 percent, more preferably at least 10 percent, and mostpreferably at least 15 percent of the amount of oxygen-18 employed inthe fiber).

Alternatively, the isotopically enriched oxygen may simply be reclaimedto a holding tank from which it can be shipped to an external facilityfor purification. The purification requirements of this embodiment areobviously less stringent than those in the one discussed above, but theseparation techniques likely could be similar to those described above.A still alternative embodiment would be to pass the separated oxygen-18to another stage in the process in which oxygen purity might not be asessential. This could be the case, for example, if an isotopic exchangewas being performed in the consolidation process on a soot blank from anOVD or VAD laydown process.

Raman amplification may be achieved using the process known asstimulated Raman scattering (SRS). In this process, photons of a shortwavelength pump are scattered by vibrational modes of the lattice of anoptical material and add coherently to the long wavelength signalphotons. When this occurs, an optical device commonly known as a Ramanfiber amplifier is created. This gain spectrum of such a device is fixedrelative to the pump wavelength by the frequency of the latticevibrations in the optical fiber.

In a typical silica optical fiber, the maximum gain occurs at afrequency shift of 13.2 THz (440 cm⁻¹) from the pump frequency. If onlya single pump wavelength is employed, the usable bandwidth of a Ramanamplifier is limited to about 20 nm in the conventional 1560 nmtelecommunications window. It is known to increase the bandwidth byusing several pump wavelengths whose gain spectra overlap. However, sucha technique requires multiple pump lasers, and the extra costsassociated with each additional laser unit.

The isotopically enriched fiber of the invention allows the bandwidth ofa Raman fiber amplifier to be broadened with the use of only a singlelaser pump. Such broadening occurs as a result of the fact thatreplacement of the lighter isotopes of either oxygen or silicon for theless abundant heavier isotopes has the affect of shifting the Ramanspectrum to the left, as is indicated by the dash lines in both FIG. 19.Note in particular that in silica formed from the natural abundance ofoxygen-16, the main peak of the spectrum occurs at a frequency shift of440 cm⁻¹ from the pump frequency. Replacing the oxygen-16 atoms withoxygen-18 shifts this peak to 420 cm⁻¹, a difference of 20 cm⁻¹. At 1560nm, the wavelength range of interest for telecommunications, thiscorresponds to a wavelength shift of 5 nm. Since the most commonly usedoptical bandwidth around the 1560 nm range is 100 nm wide, a wavelengthshift of 5 nm corresponds to approximately a 5 percent broadening of theRaman spectrum. In one embodiment, the isotopically enriched fiber ismanufactured by process in which deposited layers alternate betweenlayers of silica which are enriched with oxygen 18 (and preferablyoxygen 17 as well) and layers of silica which are not enriched withoxygen 18 (i.e., primarily conventional oxygen is employed as theoxidant for these silica layers). It is believed that such alternatinglayers will facilitate the formation of some fibers, for example, thosewhich are to be used for Raman applications. In particular, it isbelieved that the alternating layers will have a beneficial broadeningeffect on the Raman gain spectrum, as is illustrated in FIG. 20. In FIG.20, four Raman cross section curves are illustrated for various fibersin accordance with invention, namely, which employ 0, 50, and 100percent mixtures of oxygen-18 containing glass throughout theirentirely, and a fourth, average curve, which can be obtained by formingthe optical fiber preform using alternating layers of glass which employ100% oxygen-18 substitution and layers of glass which employ naturallyoccurring oxygen. As is the case with all embodiments disclosed herein,preferably, the layers of glass which contain an increased amount ofoxygen-18 also contain an increased amount of oxygen-17, in the relativeconcentrations described above (at least 5 percent, more preferably atleast 10 percent, and most preferably at least 15 percent of the amountof oxygen-18 employed in the fiber).

The aforementioned discussion applies when the isotopic substitutionoccurs between oxygen-16 and oxygen-18. Some shifting will also occur ifsilicon 28 is replaced with silicon 30. However, as the amount ofspectrum shift is substantially less (i.e. from 440 cm⁻¹ to 432 cm⁻¹ foronly a 2 percent broadening) the use of oxygen-18 enrichment is thepreferred embodiment of this particular aspect of the invention. In apreferred embodiment, this fiber is formed by depositing alternatingregions in the core of 100 percent oxygen-18 enriched silica alongsideregions of oxygen-16 silica.

The aforementioned findings indicate that the lattice motion of thesilicon molecule is dominated by the oxygen atoms, rather than thesilicon atoms. This domination indicates that the substitution ofoxygen-18 for oxygen-16 in other glass materials with a structuresimilar to silica will also result in similar Raman spectrum broadeningwhich may be advantageously exploited. In particular, oxygen-18 could besubstituted for oxygen-16 in germania (GeO₂) doped silica where thegermania is added to increase the Raman scattering coefficient and isfurther used is dispersion compensating fibers. This same principalcould be applied to optical fibers used in Raman fiber lasers andcascaded Raman resonators which are often used as high-power pumpsources for Raman amplifiers. Cascaded Raman resonators often employphosphorous doped silica (P₂O₅—SiO₂) fibers to convert the output ofytterbium doped fiber laser at a wavelength of approximately 1 nm to auseful Raman pump wavelength in the 1400 nm wavelength region. Thepresence of the phosphorous doped silica creates a strong but relativelynarrow Raman resonance at a frequency of about 40 THz and allows thewavelength conversion to be achieved in two successive frequency shifts.This invention encompasses the concept of replacing oxygen-16 and P₂O₅with oxygen-18 in order to move this resonance approximately 1.5 THz.This would allow a different range of approximately 1400 nm pumpwavelengths to be generated from the same ytterbium fiber laser source.

While this invention has been described with respect to severalpreferred embodiments, various modifications and additions to theinvention will become evident to persons of skill in the art. All suchvariations, modifications, and additions are intended to be encompasswithin the scope of this application, which is limited only by theclaims appended hereto.

1. A method for making a silica waveguide, comprising the steps of:fabricating a silica preform by heating a moving stream of vapor mixtureincluding at least one glass forming precursor including silicon and anoxidizing medium including oxygen, wherein the oxidizing medium isselected so that, when the resultant preform is used and drawn to forman optical fiber, the optical fiber includes a light conducting coreregion and at least 20 mole percent of said oxygen in said core isoxygen-18, and said core region also includes oxygen 17 in a molaramount which is at least 5 percent of the amount of oxygen 18 in saidcore.
 2. The method defined in claim 1, wherein said moving stream ofglass-forming precursor and oxidizing medium are introduced into a tube,and said tube is heated to form said preform from a glassy deposit on aninner surface of said tube.
 3. The method defined in claim 2, whereinsaid tube is heated with an isothermal heat source.
 4. The methoddefined in claim 3, wherein said heat source is an oxygen plasma heatsource.
 5. The method defined in claim 2, wherein said tube is heatedwith a substantially hydrogen-free heat source.
 6. The method defined inclaim 1, wherein only a core region of said preform is fabricated fromsaid isotopically altered oxygen and/or silicon.
 7. The method definedin claim 1, wherein the oxygen of said oxidizing medium forming saidcore portion is comprises greater than 50 mole percent oxygen-18.
 8. Themethod of claim 1, wherein said method comprises depositing layers ofsoot or glass onto a substrate, and wherein said deposited layersalternate between regions having an oxygen-18 amount which is greaterthan 20 mole percent and regions having less than 20 mole percentoxygem-18.
 9. A method of making a silica waveguide comprising the stepsof: fabricating a silica based soot preform via a chemical vapordeposition process exposing the soot preform to an atmosphere in achamber, said atmosphere comprising at least 20 percent oxygen-18, for atime and at a temperature which is sufficient to cause oxygen-18 toexchange for at least some of the oxygen-16 present in the silica soot.10. The method of claim 9, wherein said exposing the soot preform stepcomprises exposing said soot preform to a gas selected from the groupconsisting of mixture of D₂ ¹⁸O, H₂ ¹⁸O, and mixtures thereof.
 11. Themethod of claim 9, wherein said exposing the soot preform to oxygen-18step comprises exposing said soot preform at a temperature greater thanabout 1000 C.
 12. The method of claim 9, wherein said exposing stepfurther comprises recovering unused oxygen from the chamber in saidexposing step and recycling at least some of the unused oxygenatmosphere to be used in the same or another chamber in which soot isbeing exposed to oxygen-18 for a time and at a temperature which issufficient to cause oxygen-18 to exchange for at least some of theoxygen-16 present in the silica soot.